You can set up a $SIG{'__WARN__'} handler. Doing that suppresses printing the message to STDERR. See the perlvar article on %SIG for details.

What Zaxo doesn't provide, is a working example. So, here is one, tailored to your snippet:

$re = '[\w-]';
my @warnings;
{
    local $SIG{__WARN__} = sub { push @warnings, shift };
    eval {'' =~ /$re/ };
}
if($@) {
    print "error: $@\n";
} else {
    foreach my $warning (@warnings) {
        print "Got a warning: $warning";
    }
}
[download]

Alternatively, you can do your checking inside the $SIG{__WARN__} sub.

Note how you need a semicolon after some "blocks" for it to work: after the assignment of the anonymous sub to $SIG{__WARN__}, and after the eval block, at least if any statement comes right after it — which you forgot.

Perfect. Thanks to both of you. I gather that the effect of "local" as used here is to restore the original handler upon exiting the block?

I liked this enough that I bookmarked this in my delicious collection -- helped me debug a nasty problem.

I did things slightly differently though -- more localized, I think.

    {
      local $SIG{__WARN__} = sub {
        my @warnings = @_;
        foreach my $warning (@warnings) {
            print "got a warning: $warning\n";
          }
      };
      eval {'' =~ /$re/ };
    }
[download]

cheers.